This invention relates to methods for constructing high density, exceptionally complex and compact ultrasound arrays using interconnected, multi-layer structures composed of active integrated circuit devices on various planar substrates and passive devices, and more particularly, to using electrically conducting micro-bump interconnections between layers, and conductors within micro-vias for electrical connections through the semiconductor or other substrates, to realize the implementation in a single device of segregated sections or different integrated circuit technologies on a layer-by-layer basis.
Diagnostic ultrasound is an established and growing medical imaging modality. Currently one-dimensional arrays with up to 128 elements are the standard in the industry. Separate coaxial cables are used to connect the elements to the system electronics. Improved image quality requires the use of matrix (nxc3x97m) arrays with a thousand or more elements. As element numbers increase and their dimensions grow smaller, limitations to present fabrication technologies arise. Cost, ergonomics, produce-ability and reliability are important issues. Signal loss due to the capacitance of the coax cables becomes a fundamental problem.
Connecting an integrated circuit directly to the array elements alleviates all these problems. Each unit cell of such a custom Transmitter/Receiver Integrated Circuit (TRIC) may have high voltage switches for transmitting; a preamplifier which minimizes signal loss and a multiplexer to send the array signals over fewer wires. Additional signal processing and beam forming may also be included.
This disclosure first discusses the history and current state-of-the art in medical diagnostic ultrasound, emphasizing arrays, their limitations and issues.
Diagnostic ultrasound is an established, cost-effective medical imaging modality. More than 400,000 systems are in use throughout the world. This stage of development has been achieved over the last 30 years for several reasons. The equipment and exam costs are lower than competing medical imaging technologies. Structural information about normal organs and soft tissue (non-bone) and pathologies is easily obtained. Functional information such as blood flow, organ perfusion or movement of heart valves is easily obtained. The systems are portable and only require a typical examining room. Finally, it is generally considered to be non-invasive.
Medical ultrasound systems transmit a short pulse of ultrasound and receive echoes from structures within the body. The handheld probes are most often applied to the skin using a coupling gel. Specialty probes are available for endocavity, endoluminal and intraoperative scanning.
Almost all systems on the market today produce real-time, grayscale, B-scan images. Many systems include colorflow imaging. Real-time images move as the operator moves the probe (or scanhead). Moving structures, such as the heart or a fetus, are shown on the video monitor. Grayscale images depict the strength of echo signals from the body as shades of gray. Stronger signals generally are shown as bright white. Lower signals become gray and echo-free regions are black. B-scans are cross-sectional or slice images. Colorflow imaging adds a color overlay to the black and white image to depict blood flow.
Over the last 30 years, the major technical developments that have improved imaging or added diagnostic capability. Digital technology provided image stability and improved signal processing. Real-time imaging provided quicker, easier imaging and functional information. Electronically scanned linear arrays, including sequenced arrays and phased arrays, provided improved reliability. Color-flow imaging opened up new cardiac and vascular applications. Digital beamformers improved image quality. Harmonic imaging provided improved image quality particularly in difficult to image patients. Coded-excitation imaging permitted increased penetration allowing use of higher frequency ultrasound, thereby improving image contrast. Contrast agents offer improved functional information and better image quality. And 3D (volumetric) imaging presents more easily interpreted images of surfaces such as the fetal face.
One-dimensional (1D, linear or 1xc3x97m) transducer arrays, either flat or curved, having as many as 128 transducers, are the conventional electronically scanned arrays in widespread use today. Matrix arrays consisting of (nxc3x97m) elements will be required in future systems to improve image quality. All such arrays on the market today are connected to the system electronics through a bundle of coaxial cables. Beamformers in the system electronics adjust the time delays between channels to provide electronic sector scanning and focusing. High performance systems typically use all 128 elements in their beamformers. Lower performance systems may use as few as 16 of the 128 elements at any instant. The scanning function is performed by switching elements into the aperture on the leading edge of the scan and switching out elements at the trailing edge.
When a pulse is transmitted by an array, transmitter time delays on each channel are adjusted to provide a focusing effect. For reception of echoes, time delays between channels are adjusted in real-time as the pulse propagates into the body. This dynamic, or tracking focus, sweeps out from the probe into the body at the velocity of sound. Almost all ultrasound systems use dynamic focusing, which provides greatly improved resolution and image quality in the scanning plane.
1.25D arrays typically use a (128xc3x973) or (128xc3x975) matrix. They are connected to the system electronics through a similar bundle of coax cables as the 1D array. The same beamformers are also used for scanning and dynamic focusing. As the pulse propagates into the body, only the center element is initially selected for receiving the reflected signals. By switching in additional elements as the pulse propagates, the receiving aperture is enlarged and the receiver is weakly focused.
1.5D arrays use a (128xc3x97n) matrix, with n an odd number, typically 5, 7 or 9.1.5D arrays use dynamic focusing in the plane perpendicular to the scanning plane. This produces optimal resolution in all dimensions, further reducing artifacts. The key difference between the 1.25D and 1.5D arrays is the active time-delay beamforming in both dimensions. The number of elements in the elevation direction is often an odd number because elements on each side of the beam axis are electrically connected together since they both have the same time delay for on-axis targets.
1.75D arrays are very similar to 1.5D arrays with the exception that the elements in the elevation direction are individually connected to the beamformer. Limited angular beamsteering can be performed in addition to dynamic focusing. Aberration correction is also possible with the 1.75D array. These added capabilities are not present in a 1.5D array, which only provides improved focusing for on-axis targets.
2D transducer arrays are the most general type, with (nxc3x97m) elements. Dynamic focusing as well as sector beamsteering in any arbitrary direction around the axis normal to plane of the array is possible. The angles are only limited by the constraints of the beam former, the number of elements and their dimensions.
The use of larger multi-dimensional arrays introduces problems of circuit density in the supporting substrate, to a much greater extent than the linear arrays currently in use. Although the 1.25D array produces moderate improvement, the improved image quality of 1.5D and 1.75D arrays will make a quantum jump in resolution, image quality and freedom from artifacts, which suggests that resolution of the circuit density problems is very important.
All linear arrays currently on the market use piezoelectric materials as the transducing mechanism from electrical signals to ultrasound (transmitter) and ultrasound back to electrical signals (receiver). The signals are generally in the form of short pulses or tone bursts. Most high performance arrays use a piezocomposite material (FIG. 6), which is fabricated by a xe2x80x9cdice and fillxe2x80x9d technique. The piezocomposite array structure 9 provides improved bandwidth and efficiency as well as reduced crosstalk or interference between adjacent elements. Pieces of native ceramic such as Type 3203HD made by Motorola or PZT-5H made by Morgan-Matroc are diamond sawed into pillars 18. The spaces between the pillars are filled with a polymer 28 such as DER332 epoxy made by Dow Chemical. The spaces between elements 24 are often left air-filled or are filled with a sound-absorbing polymer. Top electrode 30 is the common electrical connection between elements. Bottom electrodes 22 are delineated for each array element and are used as the electrical connections to the piezocomposite material.
In a beamsteered array, the element dimensions must be about a wavelength in the steering dimension. For example, in a 3.5 MHz (1xc3x97128) array the element width is about 0.2 mm for a total array length of approximately 64 mm. In the other dimension, the element dimensions are a tradeoff between resolution and depth of focus. For a 3.5 MHz array, this dimension is 12 to 15 mm. As the frequency of the array increases, element size decreases, as does element thickness, however, the aspect ratio remains constant. Other methods of fabrication such as laser milling or scribing, etching or deposition are under development. At present, they are not well accepted.
Behind the flex circuit is a xe2x80x9cbackingxe2x80x9d 23 (FIGS. 5 and 7) that provides mechanical support and acoustical attenuation. When a piezoelectric transducer is electrically pulsed, two acoustical pulses 25 and 27 are generated that travel in opposite directions. Pulse 27 traveling out of the scanhead is desired, while pulse 25, propagating into the backing, is unwanted and is absorbed by the backing.
One or more xe2x80x9cmatchingxe2x80x9d layers 26 are next in the path of pulse 27. They serve to improve the coupling of energy from the piezocomposite into the body by matching the higher acoustical impedance piezocomposite to the lower acoustical impedance of the body. This matching layer functions in the same way as the anti-reflection coating on an optical lens. The system electronics xe2x80x9cfocusxe2x80x9d the pulse in the scanning plane 34 (the xe2x80x9cin-planexe2x80x9d) dimension.
A simple convex lens 31 forms the front surface that contacts the patient""s skin. It provides a fixed focus 33 to the sound pulse in plane 35 (the xe2x80x9cout-of planexe2x80x9d dimension), which is perpendicular to scanning plane 34.
Modern systems impose increasingly stringent requirements on arrays. Some of the important parameters that characterize typical medical ultrasound arrays include the following.
Center frequency: 3.5 MHz to 10 MHz for applications in the abdomen. The trend is towards higher frequencies to improve tissue contrast and image quality. In beamsteered arrays, the elements must be about xc2xd wavelength wide to avoid grating lobes. This results in fabrication issues with the higher frequency arrays.
Transmitter pressure output: measured as dB re 1 microPascal/Volt. Higher output for a given drive voltage is desirable.
Receiver pressure sensitivity: measured as dB re 1 microvolt/Pa. Higher receiver sensitivity is desirable.
Insertion loss: transmitter and receiver parameters are often combined into an insertion loss value in a lossless medium, i.e. (volts output)/(volts input) expressed in dB.
Fractional Bandwidth: 50% to 120% of the center frequency. Applications such as harmonic imaging demand the widest possible bandwidth consistent with maximum transmitter output and receiver sensitivity. In a B-scan image, the resolution along the direction of the pulse is a function of bandwidth. Higher bandwidth equals shorter pulses and better resolution.
Crosstalk: Crosstalk is the interference of signals between array elements. The interference may be electrical, mechanical or acoustical. It is expressed in dB re nearest neighbor. Crosstalk in a well-constructed array better than xe2x88x9230 dB.
Resolution (beam pattern): The spatial resolution of the probe in the plane normal to the beam is fundamentally a function of the ultrasound wavelength and active aperture dimension. Other factors in the system electronics, including the probe bandwidth interact with this in a complex way.
Temperature Rise: For probes that contact the human body, temperature rise is an important safety issue. Probe heating is due to internal losses in the piezoelectric materials. The maximum allowable temperature rise is 5 degrees C.
Cable parameters: As the number of wires increases, retaining cable flexibility and low weight become important to minimize operator fatigue. In addition, cable capacitance, which is about 50 pF/meter (16 pF/ft) becomes important as the element size decreases with higher frequency probes. Signal losses of over 90% are not uncommon.
Scanhead size and weight: The probe must fit a small hand comfortably for extended scanning and the weight must be a minimum to reduce operator fatigue.
Reliability: The Mean-Time-Between-Failure (MTBF) must be two years or more.
As the number of elements increases and their size decreases, however, the existing approaches may no longer be feasible or practical. Processing time, touch labor, yield, reliability and cost become limiting issues and new processes are required. With higher ultrasound frequencies and more complex matrix arrays, element size decreases. The capacitance of an element decreases linearly with the area of the element. The supporting integrated circuitry in the underlying substrate contains transmitter and receiver electronics and switching circuitry, and provides an impedance translation between the small capacitance array transducers and the higher capacitance wires in the cable.
In FIG. 9, a cross-section of a prior art piezocomposite integrated array, the array elements 18 are electrically and mechanically connected to integrated circuit (IC) substrate 32 with electrically conductive bumps 34 using metallized pads 36 on IC 32 to form a complete electrical circuit. Integrated circuit substrate 32 is typically composed of silicon, although other semiconductors may be used. Conductive bumps 34 may be composed of solder or a conductive polymer such as silver epoxy.
In addition to the impedance translating electronics, signal-processing electronics may be included in the probe to greatly simplify the fabrication of all types of matrix arrays. This has the benefit of dramatically reducing the number of wires required in the cable, but adds yet further complexity and density to the supporting integrated circuitry.
The first matrix array with an integrated circuit was developed in the early 1970""s. An 8xc3x978 element 3.5 MHz receiver array with a preamplifier integrated circuit bump-bonded directly behind each 1 mmxc3x971 mm Lithium Niobate single crystal piezoelectric element was constructed. Individual elements could be connected along a row to one of the eight output lines. Although the state of microelectronics was primitive by current standards, the array was shown to have acceptable sensitivity and demonstrated the feasibility of the approach. At that time, the diagnostic ultrasound industry was in its infancy and there was no need for such an array.
In a recent government-funded program, a real-time 3D ultrasound camera intended for Army medics to use on the front lines was designed and feasibility was proven. In this camera, an acoustical lens was used to image a volume onto a 128xc3x97128 (16,384 element) 5 MHz matrix array.
Each element of the piezocomposite array had a custom integrated circuit bump-bonded directly behind it using micro-solder balls. The piezocomposite array was air-backed, i.e. there was a small air space between the array and the IC. The bump bonds were the only mechanical and electrical connections between the array and the IC.
Each unit cell of the ROIC contained a preamplifier, signal processing, a limited amount of sampled data storage and multiplexing. The silicon was two side-buttable, permitting tiling of four pieces into the square 128xc3x97128 array. In this program, the concept of bump-bonding a matrix array directly to an integrated circuit was revalidated. Important parts of this technology are now protected by U.S. and foreign patent(s).
In another government-funded program, a camera for Navy divers to image mines in murky water was developed. The camera uses an acoustical lens as well as a second-generation 128xc3x97128 3 MHz Integrated Matrix Array. The following improvements in this new device are directly relevant to the medical ultrasound arrays. A custom Transmit and Receive Integrated Circuit (TRIC) was developed. This is the first such IC that has ever been built for an ultrasound system. The key improvement is the introduction of a 90 volt switch, enabling transmission of ultrasound pulses. The 32xc3x9764 cell IC is three-side buttable, i.e. all external connections are on one narrow edge of the silicon. This permits tiling of eight pieces of silicon to form the 128xc3x97128 array with its monolithic layer of supporting circuitry.
Moving away from the specific field of acoustical and ultrasound technologies, multi-layer printed circuit boards (PCB) have been used for many years in the electronics industry. They enable complex interconnection of multiple integrated circuits in individual packages, passive devices, connectors, etc. A conventional integrated circuit package, however, contains a silicon integrated circuit chip or die, which is wire bonded to leads that exit the package and are in turn connected to the PCB. The wire bonding together with the package require extra space as well as labor and/or further processing.
Hybrid circuit technology removes the package and wirebonds the chip directly to a passive substrate. This is very effective in miniaturizing the final assembly. However, wire bonding is still required.
The next level of complexity (or integration) is flip-chip bonding of the die directly to a passive substrate. This achieves very close to maximum chip density in a planar structure.
Recently, a further level of integration has been achieved, for example, by Irvine Sensors. Multiple layers of silicon die are sandwiched together and interconnected at the ends. This allows very high densities. There are still limitations on the circuits that may be achieved, since the interconnections are only made at the sides of the chips.
Vertical wafer feedthroughs or conductor-carrying vias for connecting an array of sensors or actuators from the transducer side to the backside of a transducer chip has been disclosed in some recent publications, as in an article entitled An Efficient Electrical Addressing Method Using Through-Wafer Vias For Two-Dimensional Ultrasonic Arrays, by Ching H. Cheng et al, of Stanford University, 2000 IEEE Ultrasonics Symposium-1179.
However, the particular ways, advantages and benefits of dividing and redistributing the traditional two dimensional topography of the supporting integrated circuitry of a small element, large scale two dimensional transducer array into segregated layers or substrates has not been heretofore explored.
The invention encompasses a new method for constructing ultra-high density, exceptionally complex and compact ultrasound arrays using multi-layer structures composed of active integrated circuit devices on various substrates and passive devices, using electrically conducting micro-bump interconnections between the substrates and micro-vias configured with electrical conductors through the substrates. This methodology allows the use of divided or different integrated circuit technologies on a layer-by-layer basis.
To this end there is provided a multilayer acoustical transducer array assembly for an ultrasound system consisting of an acoustical transducer array with a back side and back side pattern of electrical contacts. There is a first IC substrate with associated integrated circuitry for operating the array, where the first IC substrate has a first side with matching electrical contacts for contacting the transducer array for supporting electrical functions and a second side with second side electrical circuit contacts. The transducer array is aligned in a co-planar configuration with the first IC substrate, with the electrical contacts of the transducer array being electrically bonded to respective electrical contacts on the first side of the first IC substrate.
There is a final IC substrate with additional associated integrated circuitry for operating the array. The final IC substrate has a first side with matching electrical contacts and a second side. The IC substrates are aligned in a co-planar configuration, and the second side electrical contacts of each IC substrate are electrically bonded to respective matching electrical contacts on each adjacent IC substrate. Suitable connections to a power source may be provided by edge connections from one or more substrates, as is well known in the trade.
There may be at least one intermediate IC substrate with further associated integrated circuitry for operating the array, where the intermediate IC substrate has a first side with matching electrical contacts and a second side with second side electrical contacts. There may be an acoustical backing layer adhered to the second side of the final IC substrate.
The first IC substrate may have electrical paths connecting at least selected electrical contacts on its first side to respective selected electrical contacts on its second side. The electrical paths may consist of vias extending through the IC substrate, and the vias may have or be filled with electrically conductive materials.
The first IC substrate may further consist of integrated circuitry for selecting between groups of transducers within the transducer array. The groups of transducers may be orthogonally oriented lines of transducers within the transducer array.
The electrical contacts on any of the IC substrates may be configured as solder bumps, where the first side contacts of each substrate have higher reflow melting temperature than the second side contacts.
There is provided a multilayer acoustical transducer array assembly with at least one high density intermediate IC substrate with image processing capability for processing signals from the array. It may also be connected to a local or remote host computer and/or display. It has electrical connections with the associated integrated circuitry in the other IC substrates, which may include surface contacts and/or edge connections. There may be included in the assembly an integral planar display array, preferably aligned in a co-planar configuration with and electrically connected to the high density intermediate IC substrate. The intermediate IC substrate may have electrical paths connecting at least selected electrical contacts on its first side to respective selected electrical contacts on its second side, similar to other IC substrates of the assembly.
The integrated circuitry supporting the operation of the array may be divided between two or more substrates. The integrated circuitry in the first IC substrate may have at least switching circuitry for switching the transducers of the array between transmit and receive modes of operation. The integrated circuitry in the final IC substrate may have at least receiving circuitry for receiving signals from the array. Alternatively, the integrated circuitry in an intermediate IC substrate may have at least the switching circuitry for switching the array between transmit and receive modes of operation, or the receiving circuitry for receiving signals from the transducers of the array.
There is also provided a method for making a multilayer acoustical transducer array assembly including the steps of providing an acoustical transducer array with a back side and back side pattern of electrical contacts, a first IC substrate as described above, and a similar second IC substrate, then electrically bonding the matching electrical contacts on the first side of the first IC substrate to the electrical contacts of the array at a suitable pressure and a first temperature, and then electrically bonding the matching electrical contacts on the first side of the second IC substrate to the electrical contacts on the second side of the first IC substrate at a suitable pressure and second temperature, where the second temperature is lower than first temperature. This lower temperature sequence permits testing at least selected electrical functions of the first IC substrate at least in part through access to the electrical contacts on the second side of the first IC substrate before the assembly is completed.
The method may be extended to providing a third, similar IC substrate with additional integrated circuitry for supporting the operation of the array, and then electrically bonding the matching electrical contacts on the first side of the third IC substrate to the electrical contacts on the second side of the second IC substrate at a suitable pressure and third temperature, where the third temperature is lower than said second temperature. This lower temperature sequence provides for testing at least selected electrical functions of the second IC substrate at least in part through access to the electrical contacts on the second side of the second IC substrate before the assembly is completed.
Various embodiments are illustrated and described below, illustrative but not exhaustive of the scope of the invention.